A number of processes have been proposed for the vapor phase oxidation of propylene or isobutylene to form, inter alia, acrylic acid or methacrylic acid. Such processes are described, for example, in U.S. Pat. Nos. 3,065,264; 3,293,290; 3,392,196; 3,401,198; 3,428,674; and 3,475,488.
One such process which has been developed and utilized for the purpose of acrylic acid, for example, involves a multi-step procedure for the vapor phase oxidation of propylene into acrolein and acrylic acid. The prescribed multi-step operations present obvious processing problems. Moreover, the acrylic acid formed in the successive reaction zones is subject to autooxidation, resulting in relatively low product yields.
In U.S. Pat. No. 3,792,086, an improved process for the preparation of acrylic or methacrylic acids is taught, which involves a vapor phase oxidation of propylene or isobutylene at a temperature of up to 300.degree. C. and in the presence of a catalyst composition containing phosphoric acid and a catalytically effective amount of palladium metal. The acrylic or methacrylic acids are selectively produced in the single-step vapor phase process. Although this process constitutes a very significant advance in the art, it has been found that the continuous operation of the process is often accompanied by a decline of catalyst efficiency due to formation of tar which coats the catalyst preventing efficient contact with the reactants. It has now been found that the use of certain sulfur modifiers unexpectedly results in the retardation of tar formation, an extension of catalyst life and an increase in reaction rate.
Broadly, the use of a sulfur material in an oxidation process is known. For example, U.S. Pat. No. 3,009,960 teaches the oxidation of olefins such as propylene and isobutylene over a copper silicate catalyst in the presence of sulfur to produce unsaturated aldehydes. U.S. Pat. No. 2,590,124 teaches the limited oxidation of gaseous, saturated aliphatic hydrocarbons to produce oxygenated compounds, particularly aldehydes and ketones, using sulfuric acid or sulfur trioxide in the presence of a catalyst. Soviet Pat. No. 336,869 teaches the reduction of transition metal compounds, e.g., of Pd, Cu, Pt and Ru, with sulfur compounds to prepare catalysts which are used in the oxidative addition of hydrogen cyanide to produce the corresponding organonitriles. Many other examples appear in the technical literature. The present vapor phase oxidation of propylene or isobutylene to acrylic or methyacrylic acids is, however, unique and the fact that the sulfur modifiers lead to the retardation of tar formation, extension of catalyst life and increased reaction rates was quite unexpected.
Accordingly, it is the object of this invention to provide a new and improved process for the selective preparation of acrylic and methacrylic acids in substantial conversions. This and other objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description.